The manufacture of pulp from woody cellulosic materials in the Kraft process commences with the step of liberating the cellulose fibers of the wood from the lignin in which they are encased, and from the pitch which they contain. In the Kraft or other alkaline pulping process, the product of this step is a suspension of cellulose fibers more or less free from lignin in an aqueous suspension of pitch in soap form. The pitch, in hard water areas, is converted to adhesive particles by the action of calcium (and possibly by other polyvalent metal ions) which are normally present in hard water. The particles are anionic (so that they are not self-substantive to the cellulose fibers of the suspension) and are very adhesive (so that in pulp making they adhere tenaciously to metal and hydrophobic surfaces with which they come in contact). Deposition of these adhesive, metal-containing particles is particularly liable to occur in the brown stock washers, the bleaching towers, and the subsequent washers. All these and other metal and hydrophobic surfaces, prior to beating or refining, become fouled more or less rapidly with deposits of these particles, and must be cleaned regularly. The removal of these deposits from pulp-mill equipment is costly and, especially when volatile organic solvents are used, is dangerous.
To a person having skill in the art of making pulp, beating and refining have different meanings. However, for the purposes of this invention, by beating and refining, I mean the mechanical cutting or crushing of the cellulosic fibers. Therefore, reference to one term is meant to apply equally to the other.
The initial deposition of these particles is generally in the form of a rough film, and as deposition continues, thick incrustations form particularly on exposed edges to such an extent as to interfere with the operation of the pulp-mill apparatus. The composition of these particles has not been ascertained, and is complex; for example, they appear to contain pitch in free (i.e., unconverted) form. For convenience, therefore, the material of which these particles is composed is hereinafter termed "adhesive pitch".